It is generally appreciated that elemental analysis techniques have important applications to determine the elemental composition of a material in various forms. Elemental analysis techniques range from destructive (e.g.—material is destroyed in testing) to semi-destructive (e.g.—material is sampled or surface damaged) to fully non-destructive (e.g.—material is left fully intact). One view of elemental analysis is via the periodic chart to define which elements can be detected via a particular technique or device design. There are often challenges with certain elements due to interfering signals, weak signals, or the inability to cause atomic excitation. This category of techniques can include what are referred to as Inductively Coupled Plasma-Atomic Emission Spectroscopy (e.g. ICP-AES), ICP-Mass Spectrometry (e.g. ICP-MS), Electrothermal Atomization Atomic Absorption Spectroscopy (e.g. ETA-AAS), X-Ray Fluorescence Spectroscopy (e.g. XRF), X-Ray Diffraction (e.g. XRD), and Laser Induced Breakdown Spectroscopy (e.g. LIBS). Limits of detection are a key performance specification of any technique or instrument. Elemental analysis may be either qualitative (e.g. easier) or quantitative (e.g. more difficult) and often requires calibration to known standards.
As described above, the periodic table is often used to define the elements which a system can detect and quantify. A key element of interest in the analysis of metals is Carbon which is known as a “light” element according to the periodic table. For example, the carbon content of many steel compositions defines the material properties and compatibility of a particular composition with other metals. It is generally appreciated that XRF devices and in particular portable instruments are not able to reliably detect and quantify light elements such as the carbon content of a material. The defining characteristic of low carbon steel (used extensively to transport chemicals in piping) is the presence of approximately 300 ppm of carbon which would require a limit of detection of less than 100 ppm to reliably quantify (limit of quantification, LOQ, or ˜3 times LOD). Often these materials need to be tested at the point of use to confirm suitability for the purpose.
Laser Induced Breakdown Spectroscopy is an atomic emission spectroscopy technique which uses laser pulses to induce excitation. The interaction between the focused laser pulses and the sample creates plasma composed of ionized matter. Plasma light emissions can provide spectral data regarding the chemical composition of many different kinds of materials. LIBS can provide an easy to use, rapid, and in situ chemical analysis with adequate precision, detection limits, and cost. Importantly, LIBS can very accurately detect and quantify the light elements that other technologies cannot.
Laser interactions with matter are governed by quantum mechanics which describe how photons are absorbed or emitted by atoms. If an atom absorbs a photon one or more electrons move from the ground state to a higher energy quantum state. Electrons tend to occupy the lowest possible energy levels, and in the cooling/decay process the atom emits a photon. The different energy levels of different atoms produce different photon energies for each kind of atom, with narrowband emissions due to their quantization. These emissions correspond to the spectral emission lines found in LIBS spectra and their features and their associated energy levels are well known for each atom.
There are three basic stages in the plasma life time. The first stage is the ignition process which includes the initial bond breaking and plasma formation during the laser pulse. This is affected by the laser type, laser power, and pulse duration. The second stage in plasma life is the most critical for optimization of LIBS spectral acquisition and measurement because the plasma causes atomic emission during the cooling process. After ignition, the plasma will continue expanding and cooling. At the same time, the electron temperature and density will change. This process depends on ablated mass, spot size, energy coupled to the sample, and environmental conditions (state of the sample, pressure, etc.). The last stage of the plasma life is not interesting for LIBS measurements. A quantity of ablated mass is not excited as vapor or plasma, hence this amount of material is ablated as particles and these particles create condensed vapor, liquid sample ejection, and solid sample exfoliation, which do not emit radiation. Moreover, ablated atoms become cold and create nanoparticles in the recombination process of plasma.
In general there is a desire to move analytical techniques from the laboratory to the field at the point of use by using devices that are easily portable and supportable with a minimum of additional requirements. There are often significant costs and technical challenges associated with laboratory testing, in particular when there are significant time delays between the sampling and the test result, or if sampling itself presents an issue. Example markets include in-process pipe testing, scrap metal sorting, incoming material inspection, and positive material identification. Portable XRF devices have been very successful in these markets but have technical limitations in certain applications such as with the detection of light elements as described above. Further, while it is appreciated that Optical Emission Spectroscopy (e.g. OES) devices can detect light elements it is also known to be very challenging to execute in a portable form.
There are many challenges associated with bringing technologies from the laboratory to the field. In a laboratory setting, one can usually control many of the analysis variables and perform various sample preparation steps to get an accurate and repeatable result in often “ideal” conditions. Bringing the technology to the field, in particular to outdoor and often remote locations, introduces a host of variables that cannot be completely controlled. Most importantly the operating environment can vary widely including temperatures ranging from −5 to 50° C. and beyond. Additionally, sample preparation may be limited by the other tools available in the field and the technical skill of the operator. The portable instruments need to be rugged, easy to use, and minimize the amount of user intervention to get repeatable results.
Without the ability to control all of the analysis variables like in a laboratory setting, it becomes important that the portable instrument is able to operate effectively over a broad range of variables that may occur in a real field setting. The calibration of the instrument may typically occur at the factory in controlled conditions, and various factors can be intentionally altered (temperature, pressure, sample types, power settings). But, it is unlikely that every possible condition could be envisioned as well as impractical to calibrate for all potential operating conditions that may occur in the field during manufacture. Further, calibration processes typically cannot compensate for certain changes in operation due to environmental conditions, performance changes over a lifetime of use, etc. For example, the power output of a laser may vary based on a number of factors that include temperature, decrease caused by usage over time, or other factors.
Therefore, it is appreciated that there is a strong need for a portable LIBs system and methods that enable adjustment to operating conditions in order to compensate for many of the uncontrolled variables and give a result comparable to those found in a laboratory environment.